How to claim a dependent child over age 18 who:s in college

My daughter is 19 years old, lives in my home, but attends college online in another state.  How do I claim her as a dependent if she is over 18 years old?  I have provided all of her care (home, food, etc) for the entire year.  My return was kicked back because it said she was over 18.  I thought I could claim her until she was 23 if she was still in school.  Help!!